1. Field of the Invention
This invention relates generally to systems for drilling and logging boreholes for the production of hydrocarbons and more particularly to a drilling system having an acoustic measurement-while-drilling (“MWD”) system as part of a bottomhole assembly, or an after-drilling wiereline logging system having an acoustic device for measuring acoustic velocities of subsurface formations during or after drilling of the wellbores and determining the location of formation bed boundaries around the bottomhole assembly, as in the MWD system, or around the wireline logging system. Specifically, this invention relates to the imaging of bed boundaries using directional acoustic sources. For the purposes of this invention, the term “bed boundary” is used to denote a geologic bed boundary, interface between layers having an acoustic impedance contrast, or a subsurface reflection point. For the purposes of this invention, the term acoustic is intended to include, where appropriate, both compressional and shear properties.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes or wellbores are drilled through hydrocarbon-bearing subsurface formations. A large number of the current drilling activity involves drilling “horizontal” boreholes. Advances in the MWD measurements and drill bit steering systems placed in the drill string enable drilling of the horizontal boreholes with enhanced efficiency and greater success. Recently, horizontal boreholes, extending several thousand meters (“extended reach” boreholes), have been drilled to access hydrocarbon reserves at reservoir flanks and to develop satellite fields from existing offshore platforms. Even more recently, attempts have been made to drill boreholes corresponding to three-dimensional borehole profiles. Such borehole profiles often include several builds and turns along the drill path. Such three dimensional borehole profiles allow hydrocarbon recovery from multiple formations and allow optimal placement of wellbores in geologically intricate formations.
Hydrocarbon recovery can be maximized by drilling the horizontal and complex wellbores along optimal locations within the hydrocarbon-producing formations (payzones). Crucial to the success of these wellbores is (1) to establish reliable stratigraphic position control while landing the wellbore into the target formation and (2) to properly navigate the drill bit through the formation during drilling. In order to achieve such wellbore profiles, it is important to determine the true location of the drill bit relative to the formation bed boundaries and boundaries between the various fluids, such as the oil, gas and water. Lack of such information can lead to severe “dogleg” paths along the borehole resulting from hole or drill path corrections to find or to reenter the payzones. Such wellbore profiles usually limit the horizontal reach and the final wellbore length exposed to the reservoir. Optimization of the borehole location within the formation can also have a substantial impact on maximizing production rates and minimizing gas and water coning problems. Steering efficiency and geological positioning are considered in the industry among the greatest limitations of the current drilling systems for drilling horizontal and complex wellbores. Availability of relatively precise three-dimensional subsurface seismic maps, location of the drilling assembly relative to the bed boundaries of the formation around the drilling assembly can greatly enhance the chances of drilling boreholes for maximum recovery. Prior art downhole lack in providing such information during drilling of the boreholes.
Modern directional drilling systems usually employ a drill string having a drill bit at the bottom that is rotated by a drill motor (commonly referred to as the “mud motor”). A plurality of sensors and MWD devices are placed in close proximity to the drill bit to measure certain drilling, borehole and formation evaluation parameters. Such parameters are then utilized to navigate the drill bit along a desired drill path. Typically, sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a formation resistivity measuring device are employed to determine the drill string and borehole-related parameters. The resistivity measurements are used to determine the presence of hydrocarbons against water around and/or a short distance in front of the drill bit. Resistivity measurements are most commonly utilized to navigate or “geosteer” the drill bit. However, the depth of investigation of the resistivity devices usually extends to 2-3 meters. Resistivity measurements do not provide bed boundary information relative to the downhole subassembly. Furthermore, error margin of the depth-measuring devices, usually deployed on the surface, is frequently greater than the depth of investigation of the resistivity devices. Thus, it is desirable to have a downhole system which can relatively accurately map the bed boundaries around the downhole subassembly so that the drill string may be steered to obtain optimal borehole trajectories.
Thus, the relative position uncertainty of the wellbore being drilled and the important near-wellbore bed boundary or contact is defined by the accuracy of the MWD directional survey tools and the formation dip uncertainty. MWD tools are deployed to measure the earth's gravity and magnetic field to determine the inclination and azimuth. Knowledge of the course and position of the wellbore depends entirely on these two angles. Under normal operating conditions, the inclination measurement accuracy is approximately plus or minus 0.2°. Such an error translates into a target location uncertainty of about 3.0 meters per 1000 meters along the borehole. Additionally, dip rate variations of several degrees are common. The optimal placement of the borehole is thus very difficult to obtain based on the currently available MWD measurements, particularly in thin pay zones, dipping formation and complex wellbore designs.
One of the earliest teachings of the use of borehole sonic data for imaging of near-borehole structure is that of Hornby, who showed that the full waveforms recorded by an array of receivers in a modern borehole sonic tool contain secondary arrivals that are reflected from near-borehole structural features. These arrivals were used to form an image of the near-borehole structural features in a manner similar to seismic migration. Images were shown with distances of up to 18 m. from the borehole. More recently, Robbins (U.S. Pat. No. 5,678,643) disclosed use of a logging while drilling (LWD) tool for detecting the existence of and distance to adjacent bed boundaries. A transmitter assembly is used to generate either a short acoustic pulse or a swept frequency signal that is detected by an associated receiver assembly. The received signals are processed to determine the velocity of sound in the earth formation and the position of reflecting boundaries.
U.S. Pat. No. 6,084,826 to Leggett, having the same assignee as the present application and the contents of which are fully incorporated herein by reference, discloses a downhole apparatus comprising a plurality of segmented transmitters and receivers which allows the transmitted acoustic energy to be directionally focused at an angle ranging from essentially 0 degrees to essentially 180 degrees with respect to the axis of the borehole. Downhole computational means and methods are used to process the full acoustic wave forms recorded by a plurality of receivers. The ability to control both the azimuth and the bearing of the transmitted acoustic signals enables the device to produce images in any selected direction.
A problem with the prior art methods is that with the exception of Hornby, examples of images are not presented and it is difficult to estimate the resolution of the images and the distances that can be adequately imaged. Furthermore, Hornby does not address the problem of determining the azimuth of formation boundaries. As shown by the examples herein, using the method of the present invention, a greater depth of penetration can be obtained than that possible using prior art methods. In addition, the azimuthal images can be obtained at any desired azimuth: this is in contrast to Leggett where the sources and/or receivers have to be beam-steered to a preselected azimuth, and if an image at a different azimuth is desired, the acquisition has to be repeated.